The invention relates to high tear strength, low compression set, low cost heat curable silicone elastomers.
Silicone heat curable elastomers (HCE) generally consist of diorganopolylsiloxane gum, fluid and silica filler. Silica filler acts as a reinforcing component to provide mechanical strength. Generally, fumed silica based HCE compounds provide good mechanical properties with intermediate tear strength and poor compression set.
Current HCE compounds are typically molded and produce products exhibiting excellent tear resistance with compression sets greater than 70%, thus requiring post baking to reduce the compression set to less than 40%. It would also be a great benefit to obtain an addition cure HCE with low compression set immediately upon molding while maintaining excellent tear strength and Shore A hardness. It would be a great benefit to obtain a HCE system offering low cost, good mechanical properties, high tear strength and low compression set upon curing, eliminating the need for post baking.
The present invention is directed to a curable composition comprising: (a) an alkenyl terminated linear diorganopolysiloxane gum; (b) an alkenyl containing diorganopolysiloxane gum; (c) a precipitated silica reinforcing filler with surface area of from about 90 to 300 m2/g; (d) a hydroxy terminated polysiloxane fluid; (e) an organohydrogenpolysiloxane; (f) a low compression set additive; (g) an addition-cure catalyst, and to a silicone elastomer formed by curing the composition. The cured silicone elastomer has high tear strength and low compression set upon curing and is produced at a lower cost.
Precipitated silica filler based HCE compounds provide excellent compression set but traditionally exhibit poor tear strength. The precipitated silica filler offers significant cost savings versus fumed silica filler. When used in combination with the low compression set additive of the present invention, a HCE compound providing high tear strength and low compression set is produced.
In a preferred embodiment, the composition of the present invention comprises, based on 100 parts by weight of the composition, from 60 to 98 parts by weight (xe2x80x9cpbwxe2x80x9d), even more preferably from 75 to 95 pbw, of the alkenyl terminated diorganopolysiloxane gum; from 2 to 40 pbw, even more preferably from 15 to 25 pbw, of the alkenyl-containing diorganopolysiloxane gum; from 10 to 200 pbw, even more preferably from 20 to 80 pbw, of the precipitated silica; from 0.1 to 10.0 pbw, even more preferably from 0.5 to 5.0 pbw of the hydroxy terminated polysiloxane; 0.1 to 30 parts by weight of the organohydrogenpolysiloxane; an effective amount of the low compression set additive; and an effective amount of a catalyst.
Compounds suitable as the alkenyl terminated gum component of the composition of the present invention include, for example, vinyl, propenyl, and butenyl terminated gums. In a preferred embodiment, the alkenyl terminated diorganopolysiloxane gum is a vinyl terminated dialkylpolysiloxane gum, more preferably a vinyl terminated dimethylpolysiloxane gum or a vinyl terminated poly(dimethylsiloxane-co-methylvinylsiloxane).
In a preferred embodiment, the alkenyl terminated diorganopolysiloxane is one according to the formula:
MviDxDviyMvi
where Mvi is R1R22SiO3/2 
D is R32SiO2/2;
Dvi is R4R5SiO2/2;
where R1 and R4 are each independently (C2-C6)alkenyl, preferably vinyl, R2, R3 and R5 are each independently (C1-C6)alkyl or (C2-C6)alkenyl, preferably (C1-C6)alkyl, more preferably methyl, and x and y are chosen so that the viscosity of the gum is in the range of from about 1,000,000 to about 200,000,000 centipoise at 25xc2x0 C., and having an alkenyl concentration of about 0.001 to about 0.01 mole percent of siloxy units. In one preferred embodiment, preferably, y is 0.
As used herein, xe2x80x9c(C1-C6)alkylxe2x80x9d means a straight or branched chain alkyl group containing from 1 to 6 carbon atoms per group, such as, for example, methyl, ethyl, propyl, and butyl.
As used herein, xe2x80x9c(C2-C6)alkenylxe2x80x9d means a straight or branched chain alkenyl group containing from 2 to 6 carbon atoms per group and at least one double bond between two carbon atoms per group, such as, for example, vinyl, propenyl and butenyl.
Compounds suitable as the alkenyl-containing diorganopolysiloxane gum component of the present composition include, for example, vinyl-, propenyl- and butenyl-containing gums. In a preferred embodiment, the alkenyl units are on-chain. As used herein, xe2x80x9con-chainxe2x80x9d means that the alkenyl units are on di-organo-functional siloxane units of the gum. In a preferred embodiment, the alkenyl-containing diorganopolysiloxane gum is a vinyl-containing dialkylpolysiloxane, preferably a vinyl-containing dimethylpolysiloxane, even more preferably trimethyl-terminated poly(dimethylsiloxane-co-methylvinylsiloxane).
In a preferred embodiment, the alkenyl-containing diorganopolysiloxane gum is one according to the formula:
MDwDvizM
where M is R63SiO3/2,
D and Dvi are as previously described, each R6 is independently (C1-C6)alkyl or (C2-C6)alkenyl, preferably (C1-C6)alkyl, more preferably methyl, where w and z are chosen so that viscosity ranges from about 100,000 to about 200,000,000 centipoise at 25xc2x0 C. and having an alkenyl concentration of from about 0.5 to about 15 mole percent of siloxy units.
Compounds suitable as the non-fumed silica filler are those that have reinforcing properties for silicone elastomers, such as precipitated silicas and silica gels, preferably precipitated silicas. At least part of the non-fumed silica filler must be a precipitated silica reinforcing filler having a surface area of from about 90 to about 300 m2/g. Non-fumed silica fillers are known in the art and are commercially available. An example of a non-fumed silica filler suitable for use in the present invention is DeGussa FK-140(copyright) precipitated silica, commercially available from DeGussa (New Jersey).
In a preferred embodiment, the hydroxy terminated polysiloxane is one according to the structural formula:
xe2x80x83MDaDvibM
where M is R7R62SiO3/2,
D and Dvi are as previously defined such that the alkenyl content is from 0 to about 2.0 mole percent, each R6 is as previously defined, R7 is OH, a and b are chosen such that the viscosity is from about 25 to about 40 centistokes at 25xc2x0 C.
In a preferred embodiment, the hydride-containing polysiloxane is one according to the structural formula:
MHDcDdHMH
where MH is R8R62SiO3/2,
DH is R9R6SiO2/2, each R6 is as previously defined, R8 and R9 are each H, and c and d are chosen such that the viscosity is from about 10 to about 1000 centipoise at 25xc2x0 C. and the hydride content is from about 0.05 to about 5.0 percent by weight, subject to the limitation that the molar ratio of the hydride and vinyl content in the composition must be greater than or equal to 4, preferably greater than 6, more preferably from about 6 to about 11.
Compounds suitable as the low compression set additive are those that effectively reduce compression set of the cured silicone elastomer, inhibit cure at room temperature and prevent unwanted cure. Examples of the low compression set additive for use in the present invention include, but are not limited to, acetylene alcohols such as 1-ethynl-1-cyclohexanol (xe2x80x9cECHxe2x80x9d), 9-ethynyl-9-fluorenol, and the like, preferably ECH and 9-ethynyl-9-fluorenol, and peroxide inhibitors such as methylethylketone peroxide, and the like, preferably, methylethylketone peroxide. The acetylene alcohols and peroxide inhibitors are known in the art and are available commercially.
In a preferred embodiment, the low compression set additive is an acetylene alcohol having the formula:
Hxe2x80x94Cxe2x89xa1Cxe2x80x94R10xe2x80x94OH
wherein R10 is a divalent hydrocarbon radical comprising from 6 to 40 carbon atoms where the structure of R10 may be any combination of linear, branched, aliphatic, aromatic, cycloaliphatic and olefinic, with the limitation that the alcohol is always xcex1 to the acetylene group. Preferably, the low compression set additive is 1-ethynyl-1-cydohexanol or 9-ethynyl-9-fluorenol.
Compounds suitable as the catalyst in the present invention are known in the art. Examples of effective catalysts for use in the present invention are platinum and complexes of platinum that provide at least 0.1 parts per million of platinum in terms of platinum metal. Examples of suitable platinum catalysts include, but are not limited to, platinum black, chloroplatinic acid, alcohol modified chloroplatinic acid, and complexes of chloroplatinic acid with olefins, aldehydes, vinylsiloxanes or acetylene alcohols. A preferred catalyst for use in the present invention is a Karstedt catalyst.
Other optional additives used in the compositions of the present invention are coloring agents and pigments, and standard additives known in the art to improve heat aging and oil immersion, such as iron oxide, extending fillers such as micro mica, cerium hydroxide, and the like.
The process for forming this composition can be either cold mix or hot mix at temperature above 100xc2x0 C. The cold mix can be achieved using Banbury or Doughmixer equipment. The hot mix can be achieved using either a Doughmixer batch process or an extruder-type continuous process.
The composition of the present invention is used primarily as a heat curable rubber in applications requiring high consistency and malleable compositions. Curing is generally high temperature curing, i.e., at a temperature of about 100xc2x0 C. or higher.
The heat curable silicone elastomer composition of the present invention may be used in a variety of applications, including, for example, automotive gaskets, electronic keyboards, consumer goods such as baby bottle nipples, and the like.
The examples given below are given for the purpose of illustrating the present invention. All parts are by weight. The following chart describes the components used in the examples:
Axe2x80x94vinyl terminated polydimethylsiloxane gum (viscosity 100,000 to 200,000,000 cps)
Bxe2x80x94dimethylvinyl terminated poly(dimethylsiloxane-co-methylvinylsiloxane) (viscosity 1,000,000 to 200,000,000 cps)
Cxe2x80x94trimethyl-terminated poly(dimethylsiloxane-co-methylvinylsiloxane) (viscosity 1,000,000 to 200,000,000 cps)
Dxe2x80x94hydroxy-terminated polydimethylsiloxane (viscosity 25 to 40 centistokes)
Exe2x80x94hydride containing polysiloxane (35 to 75 centistokes)
Fxe2x80x94Fillerxe2x80x94non-fumed, precipitated silica filler (90-300 m2/g hydrophilic silicon dioxide)
Gxe2x80x94low compression set additivexe2x80x949-ethynyl-9-fluorenol
Hxe2x80x94low compression set additivexe2x80x94ECH
Ixe2x80x94Pt catalyst (10% platinum in divinyltetramethyldisiloxane)